in adults at which the quantity of absorbed zinc is equal to the total endogenous zinc losses. This critical level, derived from all available sets of data for adult men, yields an average excretion of 34 μg/kg/ day of zinc and is used for children beyond 1 year of age and adolescents. Therefore, 50 μg/kg/day is used for older infants and 34 μg/ kg/day for children aged 1 through 3 years. It is recognized that this is an approximation, not only because of the extrapolation of values but also because intestinal excretion of endogenous zinc is strongly correlated with zinc absorption.
Urinary losses of zinc are approximately 7.5 μg/kg/day for both men and women (see “Adults Ages 19 Years and Older”). After early infancy, excretion rates for children on a body weight basis seem to differ very little from adult values (Krebs and Hambidge, 1986). No data are available on the integumental losses in children, so estimates for children are derived from data in adult men (Johnson et al., 1993), which provide an estimate of 14 μg/kg/day of zinc. Therefore, the estimated total endogenous excretion of zinc is 64 μg/kg/ day for older infants and 48 μg/kg/day for children aged 1 through 3 years.
Requirements for Growth. These requirements have been estimated from chemical analyses of infants and adults, which give an average concentration of 20 μg/g wet weight of zinc (Widdowson and Dickerson, 1964). It is assumed that each gram of new lean and adipose tissue requires this amount of zinc. The average amount of new tissue accreted for older infants and young children is 13 and 6 g/ day, respectively (Kuczmarski et al., 2000).
With the estimates above, the total amount of absorbed zinc required for infants ages 7 through 12 months is 836 μg/day (Table 12-2). The corresponding value for children ages 1 through 3 years is 744 μg/day (Table 12-3).
Fractional Absorption of Dietary Zinc. Fractional absorption probably has the greatest variation of any of the above physiological factors, depending as it does on numerous factors including quantity of ingested zinc, nutritional status, and bioavailability. Although a “critical” average fractional absorption of 0.4 has been derived from the data sets used for adult men (see “Adults Ages 19 Years and Older”), a more conservative value of 0.3 is used for preadolescent children. This value is based on studies of infants and young children reported by Fairweather-Tait and coworkers (1995) and Davidsson and coworkers (1996). To calculate the dietary zinc requirement based on